A 2014 study from Northwestern University’s school of medicine finds that the widespread switch from butter to soybean, corn and canola oils in the U.S. population may be decreasing lung function in as many as 4.5 million Americans. Consumption of these oils may even be linked to the much greater incidence of asthma today.

At the center of these provocative findings are two different forms of vitamin E: gamma-tocopherol and alpha-tocopherol. While alpha-tocopherol, found in olive and sunflower oils, was shown to protect against lung inflammation and hyper-responsiveness, the gamma-tocopherol found in soybean, corn and canola oils had a contrary effect.

This form of tocopherol was linked with lung inflammation, first in studies with mice and then in an analysis of data collected from 4,500 young adults who were part of a coronary risk study.

Dr. Joan Cook-Mills and her team at the Feinberg School of Medicine examined both the types of amounts of tocopherol present in these individuals’ blood plasma and their lung function. Those who had a high level of gamma-tocoperol were found to have a 10 to 17 percent reduction in lung function.

“A 10 percent reduction in lung function is like an asthmatic condition,” explained Cook-Mills, an associate professor of medicine in allergy and immunology. “People have more trouble breathing. They take in less air, and it’s harder to expel. Their lungs have reduced capacity.”

The better news was the beneficial effects of olive and sunflower oils.

“People in countries that consume olive and sunflower oil have the lowest rate of asthma and those that consume soybean, corn and canola oil have the highest rate of asthma,” said author, whose study was published in the journal Respiratory Research. “When people consume alpha-tocopherol, which is rich in olive oil and sunflower oil, their lung function is better.”

Based on the rate of those affected in her study, Cook-Mills estimates there could be 4.5 million individuals in the U.S. with reduced lung function as a result of high gamma-tocopherol consumption.